1. Field of the Invention
The present invention relates to paper making and, more particularly, pertains to a new method and apparatus suitable for drying a paper web.
2. Description of the Prior Art
Current industrial practice is to manufacture paper by successively forming pressing and drying a moist fibrous web. This process is characterized by a relatively high energy consumption which accounts for much of the capital and operating cost in papermaking.
One problem resides in the fact that known suction rolls shed vacuum at each turn. For a sheet as porous as newsprint, for example, the air volume shed from drill holes defined in the peripheral surface of the suction roll is about equal to the air drawn through sheet pores. Therefore, half the vacuum pump energy is lost.
A second problem is a discharge of the vacuum pumped, to the atmosphere, thereby wasting the heat of vapor compression.
A third problem is that known suction roll have a centrifugal throw of moisture toward the sheet, at the sheet exit from the roll, undoing some drying just done.
A fourth problem is moisture removal by intermittent forces, including foil pulses, suction and press pulses, heat pulses, each force followed by a lapse of drying force, i.e. ineffective travel. Moreover, surface tension and capillary action let moisture migrate backward, a type of rewetting which undoes drying just done.
A fifth problem occurs during sheet transfers, disturbing fiber bonds and sheet dimensions because the sheet is damp and weak.
A sixth problem is a need to synchronize speeds between machine sections requiring controls which are costly to buy and maintain.
Another problem is the loss of drying heat to the atmosphere thereby wasting fuel and investment in a steam plant.